A conventional weight detection circuit for the load cell scale, as shown in FIG. 1, provides an output of a bridge circuit 1 of a strain gauge resistance of the load cell, that is, an analog voltage proportional to weight, to a high-input-impedance differential amplifier 2 (e.g., an instrumentation amplifier), the output thereof being provided to an A-D converter 4 through a bias circuit 3. In addition, resistances R.sub.0 at the bridge circuit 1 serve as temperature-compensating resistances for the load cell.
Here, an output voltage V.sub.out of the high-input-impedance differential amplifier 2 is represented by the following equation: ##EQU1## where V.sub.1, V.sub.2 are output voltages of bridge circuit 1, and a is an amplification factor of the high-input-impedance differential amplifier 2.
A node equation is set up at each node of bridge circuit 1 and solved to obtain the following equation: ##EQU2## where R is the strain gauge resistance at zero weight,
.DELTA.R is a change in resistance proportional to weight PA1 V.sub.a, V.sub.b are present on applied voltage, and PA1 R.sub.0 is a temperature compensating resistance.
When Equation 2 is substituted into Equation 1, the following equation is obtained ##EQU3##
In a case that the weight detection circuit for the load cell scale is constructed as the above, since the output of bridge circuit 1 is amplified by the high-input-impedance differential amplifier 2, the circuit 2 is substantially isolated, so that the temperature characteristics of (a) four strain gauge resistances (R.+-..DELTA.R) at the bridge circuit 1, (b) a feedback resistance R.sub.f and an input resistance R.sub.i at the high-input-impedance differential amplifier 2, (c) two voltage-dividing resistances R.sub.d1 and R.sub.d2 thereof, and (d) three resistances R.sub.b1, R.sub.b2 and R.sub.b3 at the amplifier 2, need to be considered only within each group (a), (b), (c) or (d), but need not be considered together for the whole circuit thereby facilitating the design. However, the number of parts in such a circuit increases so that the conventional weight detection circuit is defective in a high manufacturing cost.